Describe post-transcriptional processing of RNA in eukaryotes
DNA to RNA- Transcription. During this process, a complementary MRNA strand is formed from
a DNA strand.
Post-transcription processing of RNA is eukaryotes takes place as follows:
1)RNA to protein-Translation. In the translation process, the coded information coming originally from DNA, contained in mRNA, programmes ribosomes for the synthesis of protein molecules.
2) Chain initiation and chain termination codons; AUG (methionine) and GUG (valine) triplets are designated as chain initiation codons, while UAA, UAG (tyrosine) and UGA (tryptophan) triplets, as chain termination codons
3) Activation of amino acids and transfer of activated amino acid to -tRNA. First, an amino acid
molecule is activated by ATP in the presence of a specific activating enzyme (called amino acyl
-tRNA synthetases). This results in the formation of amino acryl-adenylic acid compound (amino
acyl-AMP-E) and release of pyrophosphate (PP). Therefore the amino acyl-AMP-enzyme complex, formed during the first step reacts with a particular -tRNA and transfer the amino acid to the -tRNA. The specific amino acid is attached to the 3'-end of specific -tRNA molecules.
4) Role of the ribosome in protein synthesis: Once the amino acids have become attached to their respective -tRNA, they diffuse to ribosomes where protein synthesis actually occurs. Ribosomes have two sites for binding amino acyl-tRNA-(i) aminoacyl (or A site), and (ii) peptidyl (or P site). The initiating formylmethionine -tRNA can bind only with P site, whereas all other newly coming aminoacyls -tRNA bind to A site
5)The initiation of polypeptide chain; It is always brought about by the amino acid methionine and, therefore, methionyl t-RNA, charged with methionine binds to the initiator codon on the mRNA of the P site.
6) Elongation of polypeptide chain; A second tRNA charged with an appropriate amino acid forms hydrogen bonds with the second codon on mRNA at the A-site of the ribosome. While the two tRNA molecules are held, a peptide bond forms between the last amino acid in the polypeptide chain and the amino acid held by the tRNA at the A-site. The tRNA at the P-site then releases the
polypeptide chain and leaves the ribosome. At this point, the ribosome shifts so that the remaining tRNA (formerly at the A-site) is moved to the P-site, which leaves the A-site free to bind with the tRNA carrying the next amino acid for the chain. This process is known as translocation. It results in the third codon coming into the A-site and an appropriate tRNA charged with a third amino acid would bind at the A-site. The process of peptide bond formation and translocation is repeated.
7) Termination and release of polypeptide chain: A termination codon (UUA, UAG and UGA) is
not recognised by the anticodons of any of the normally occurring aminoacyl-t RNAs, and its
presence in the decoding or aminoacyl site precludes the addition of any further amino acids to the chain and this protein synthesis is stopped. The terminating codons are recognised by some release factors which help in the splitting of the carboxyl end of the polypeptide and the last tRNA carrying the chain. The completed polypeptide chain is finally released in the cytoplasm and undergoes various other modifications. After the synthesis of protein, mRNA degenerates. The ribosomes and tRNAs on release from the mRNA can function again in the formation of another polypeptide.
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